SPECIAL TOPICS
TESTS FOR CHANGES IN FLOW REGIMES Hege Hisdal
7.4 Detection of changes
Changes in hydrological regimes may be studied analysing historical data. Several studies of mean seasonal or monthly discharge exist. Stolte & Herrington (1984) give an example where explanations for changes in a Canadian river flow regime are sought in precipitation and evapotranspiration changes and shifts in land use and agricultural practices. Both parametric and non-parametric tests were used to detect changes in monthly flow. There are several studies of long river flow series from the Nordic countries. An example including seasonal patterns, is the extensive mapping of the behaviour of river flow in time and space given in Hisdal et al. (1995). Arnell et al. (1990) describe the impact of climate variability and change
on river flow regimes in the UK and similar studies can be found for other countries or single river basins (e.g. Wateren-de Hoog, 1995).
Another approach is the modelling of river flow under the assumption of changes or fluctuations in the climate. Nemec & Schaake (1982) utilised a deterministic conceptual rainfall-runoff model to estimate the sensitivity of water resources systems to climate variations. Other examples of impact assessments using rainfall-runoff models are given in Arnell (1992 b), Krasovskaia & S (1997) and S (1998). The latter describes expected changes in runoff regimes in different altitude zones (mountain, lowland etc.) due to alternative future global change scenarios and the resulting impacts on the Nordic system for hydroelectric power production.
Fig. 7.2. Smoothed seasonal mean regional discharge values from north-western Norway.
The values are given as a percentage deviation from the long-term mean value.
Fig. 7.3. Trend in seasonality for POT (station Spey, UK). The water year in shown on the x-axis whilst the angle representing the season is shown on the y-x-axis, confidence internals for
In general, the exploratory/visual analyses described in Chapter 4 and the tests for changes recommended in Chapter 5, can be used to detect changes in river flow regimes. Fig.
7.2 shows an example of smoothed mean seasonal flow in north-western Norway (Hisdal et al., 1995).
An important supplement is to regard the timing of seasonal events as circular data (especially recommended if the event occurs around the turn of the year). Different statistical methods for displaying and testing for changes in such data are described in Fisher (1993).
A trend test for circular data is applied to detect changes in the timing of UK floods (Robson et al., 1996). The timing of the flood peak is represented by an angle ranging between 0 and 2π. The angle representing the mean day of occurrence, µ, is modelled as a function of time as:
µt = µ +2tan-1(βt) (7.4)
where µ=2π(DayNo+0.5)/(days in year), t is the year, β represents the trend component and tan-1 is link function with the property of mapping the real line to (-π, π). Full details of the test may be found in Fisher (1993). An example of a plot showing trends in the timing of UK peak over threshold floods (POT) is shown in Fig. 7.3.
The sensitivity of the stability of river flow regimes to small fluctuations in temperature using historical temperature series for Scandinavia is described in Krasovskaia (1996) and Krasovskaia & Gottschalk (1997).
The use of statistical tests to detect changes in entropy has not yet been utilised. A topic for further research would therefore be the applicability of different statistical tests to detect temporal and spatial changes in entropy.
References
Arnell, N., 1992 a. Impacts of climatic change on river flow regimes in the UK. J. of Inst. of Water and Environ. Manag., 6(4), 432-442.
Arnell, N., 1992 b. Factors controlling the effects of climate change on river flow regimes in a humid temperate environment. J. of Hydrol., 132, 321-342.
Arnell, N., Brown, R.P.C. & Reynard, N.C.S., 1990. Impact of climate variability and change in river flow regimes in the UK. Report no. 107, Institute of Hydrology, Wallingford, UK.
Arnell, N.W., Krasovskaia I. & Gottschalk, L., 1993. River flow regimes in Europe. In: Flow Regimes from International and Network Data ed. by A. Gustard, Vol I, Hydrological Studies. Institute of Hydrology, Wallingford, UK, 112-121.
Fisher, N.I., 1993. Statistical Analysis of Circular Data. Cambridge University Press.
Gottschalk, L., Jensen, J.L., Lundquist, D., Solantie, R. & Tollan, A., 1979. Hydrological regions in Nordic countries. Nordic Hydrol. 10, 273-276.
Haines, A.T., Finlayson, B.L. & McMahon, T.A., 1988. A global classification of river regimes, Appl. Geogr., 8, 255-272.
Hisdal, H., Erup, J., Guómundsson, K., Hiltunen, T., Jutman T., Ovesen, N.B. & Roald, L.A., 1995. Historical runoff variations in the Nordic countries, NHP Report No 37.
Krasovskaia, I., 1995. Quantification of the stability of river low regimes. Hydrol. Sci. J., 40(5), 587-598.
Krasovskaia, I., 1996. Sensitivity of the stability of river flow regimes to small fluctuations in temperature. Hydrol. Sci. .J., 41(2), 251-264.
Krasovskaia, I., Arnell, N. & Gottschalk, L., 1994. Flow regimes in northern and western Europe: development and application of procedures for classifying flow regimes. In:
FRIEND: Flow Regimes from International and Network Data, ed. by P. Seuna, A. Gustard, N.W. Arnell & G.A. Cole, Proc. 2nd. FRIEND Conf. UNESCO, Braunsweig, Germany, October 1993, 185-193, IAHS Publ. no. 221.
Krasovskaia, I. & Gottschalk, L., 1992. Stability of river flow regimes. Nordic Hydrol., 23, 137-154.
Krasovskaia, I. & Gottschalk, L., 1997. Stability of river flow regimes. 1n FRIEND Flow Regimes from International Experimental and Network Data, Third Report: 1994-1997, ed.
By G. Oberlin, Cemagref, Lyon, France, 79-86.
Krasovskaia, I., Gottschalk, L., Arnell, N. & Wateren-de Hoog, B., 1993. Variations in regime class over time. In: Flow Regimes from International and Network Data, ed. by A.
Gustard, Vol. I, Hydrological Studies. Institute of Hydrology, Wallingford, Oxfordshire, UK, 13 1-138.
Krasovskaia, I. & S N.R., 1997. Sensitivity of the stability of Scandinavian river flow regimes to a predicted temperature rise. Hydrol. Sci. J., 42(5), 693-711.
Lvovich, M.I., 1938. Opyt klassifikatsii rek SSSR (Experience of classification of rivers of the USSR). Trudy (3(316, Leningrad (in Russian).
Nemec, J. & Schaake, J., 1982. Sensitivity of water resources systems to climate variation.
Hydrol. Sci. J., 27, 327-343.
Pardé, M., 1955. Fleuves et Rivières. Collin, Paris.
Robson, A. & Reed, D.W., 1996. Non-stationarity in UK flood records. Flood Estimation Handbook Note 25, Institute of Hydrology, Wallingford, UK.
Stolte, W.J. & Herrington, R., 1984. Changes in the hydrologic regime of the Battle river basin. Alberta, Canada, J. of Hydrol., 71,285-301.
Saelthun N.R. (ed.), 1998. Climate change impacts on runoff and hydropower in the Nordic countries. Nordic Council of Ministers, TemaNord 1998, 552.
Ward, RC., 1984. Some aspects of river flow in northern New South Wales, Australia. J. of Hydrol., 71, 31-51.
Wateren-de Hoog, B., 1995. The effect of climate variability on discharge as dependent on catchment characteristics in the Upper Loire basin, France. Hydrol. Sci. J., 40(5), 633-646.
CHAPTER 8
SPATIAL/REGIONAL TRENDS